The present invention relates to an electrolytic capacitor, and more specifically, to an electrolytic capacitor with an improved sealing structure.
(1) Conventional Electrolytic Capacitor
As shown in FIG. 22, the electrolytic capacitor structurally comprises capacitor element 20 formed by winding both anode and cathode electrode foils connected with lead terminal 10 as electrode drawing unit via a separator, and outer case 30 made of aluminium and a rigid resin in a bottomed cylindrical shape, where the capacitor element 20 is sealed. In this case, the lead terminal 10 is drawn outside through through-hole 41 of sealing body 40 fitted on the inner periphery of the opening of the outer case 30. Herein, rubber of a low elasticity modulus has been used as the sealing body 40, from the respect of retaining the air tightness between the lead terminal 10 passing through the sealing body 40 and the outer case 30. As shown in FIG. 23, alternatively, the lead terminal 10 comprises plain portion 11 connected to an electrode foil, round rod portion 12 for passing through the sealing body, and external connection portion 13.
Electrolytic capacitor as shown in FIGS. 22 and 23 is generally produced by the following procedures. As shown in FIG. 23, more specifically, a high-purity aluminium foil in a band shape is subjected to a chemical or electrochemical etching process so that the surface of the aluminium foil can be enlarged; and then, the aluminium foil is subjected to a chemical process in chemical solutions such as aqueous ammonium borate solution. Thus, anode electrode foil 21 with an oxide film layer formed on the surface thereof and cathode electrode foil 22 made of the high-purity aluminium foil singly processed with etching are prepared. As shown in FIG. 23, then, the plain portions 11, 11 of a pair of the lead terminals 10, 10 are individually connected to these anode electrode foil 21 and cathode electrode foil 22; then, the resulting both anode and cathode electrode foils 21, 22 are wound via a separator made of manila paper and the like, to form the capacitor element 20 (element formation process).
Subsequently, the formed capacitor element 20 is impregnated with an electrolyte solution for driving electrolytic capacitor (electrolyte solution impregnation process). Continuously, the capacitor element 20 is placed and sealed in the outer case 30 in the bottomed cylindrical shape (assembly process). At the assembly process, firstly, the lead terminal drawn out of the capacitor element 20 is inserted in the through-hole of the sealing body 40, thereby moving the lead terminal 10 relative to the sealing body 40; by subsequently drawing the whole external connection portion 13 of the lead terminal 10 from the through-hole, the round rod portion 12 of the lead terminal 10 is fixed in the through-hole of the sealing body 40. As shown in FIG. 22, then, the capacitor element 20 is placed in the outer case 30; after the sealing body 40 is fitted on the opening of the outer case 30, subsequently, the opening of the outer case 30 is subjected to a drawing process, which serves to seal the outer case 30 (assembly process).
(2) Conventional Sealing Body
As the sealing body 40 for sealing the opening of the outer case 30 in such conventional electrolytic capacitor as described above, rubber at a high elasticity modulus has been used from the respect of retaining the air tightness between the lead terminal 10 through the sealing body 40 and the outer case 30. Because rubber has a larger gas permeability constant, the electrolyte solution permeates through the rubber and is dispersed outside or exogenous gas (water, oxygen and the like) infiltrates into the inside of the case, when the rubber is used as the sealing body. Accordingly, the capacitor performance has thereby been deteriorated.
So as to prevent such occurrence, the use of a metal material or a rigid resin or the like with a small gas permeability constant as the sealing body is suggested. Because materials with small gas permeability constants generally have large elasticity moduli and are of high rigidity compared with rubber, it is difficult to retain the air tightness of the contact portion between the lead terminal passing through the sealing body and the outer case.
As shown in FIG. 22, therefore, a technique has been developed, so as to attain the reduction of the gas permeability and the enhancement of the air tightness of the contact portion between the lead terminal and the sealing body by drawing the lead terminal from the elastic body, and the technique comprises constituting a sealing body by bonding a tube-like elastic body made of rubber or a fluorine resin or the like to a sealing plate made of a rigid resin or the like or imbedding the tube-like elastic body in the sealing plate and drawing the lead terminal from the elastic body. Additionally, such technique is disclosed in for example Japanese Utility Model Laid-open Nos. 7317/1980, 115041/1980 and 132936/1980.
(3) Problems of Such Sealing Structure
As mentioned above, however, the elastic body bonded to the sealing plate or integrally imbedded in the sealing plate can have high shape stability but is relatively not readily deformable.
In conventional elastic bodies integrated with sealing plates with high rigidity, the deformation of the elastic bodies along the longitudinal direction is disturbed by the sealing plates and therefore, the elastic bodies cannot elongate along the longitudinal direction, even when the round rod portion of the lead terminal applies pressure to the elastic bodies during the insertion of the lead terminal through the elastic bodies. Consequently, the elastic bodies are compressed, leading to the volume reduction and the emergence of high stress, so that a high insertion pressure is disadvantageously applied to the lead terminal. Additionally, the elastic bodies cannot be made of materials with high elasticity moduli, involving large gas permeability constants. Hence, gas permeation can never be reduced.
So as to reduce the pressure during the insertion of the lead terminal, alternatively, the inner diameter of an elastic body can be almost equal to the outer diameter of the round rod portion of the lead terminal. In that case, however, slight error in the dimension of the elastic body occurs in a dependent manner on the processing precision and dimensional precision of the elastic body, which reduces the air tightness between the elastic body and the lead terminal, involving the enlargement of the variation of the life profile, disadvantageously.
On contrast, a modification of the production method of sealing bodies enables the procurement of a higher processing precision and a higher dimensional precision than conventional ones, but such modification induces the reduction of the productivity and also raises the production cost, undesirably.
Furthermore, the conventional methods comprising bonding an elastic body to the inner face of the through-hole of a sealing plate or imbedding an elastic body in the inner face thereof during the molding of the sealing plate requires complicated procedures, disadvantageously, leading to the reduction of the productivity.
(4) Problems due to Electrolyte Solution
Various types of electrolyte solutions for the impregnation of capacitor element 3 and for driving electrolytic capacitor have been known, and the performance of an electrolyte capacitor depends on the electrolyte solutions used therein. Among them, an electrolyte solution using y- butyrolactone as the principal solvent and so-called quaternary ammonium salt as the dissolving substance, namely a salt comprising tetraalkylammonium ion as the cation component and an acid-conjugated base as the anion component has been known.
The electrolyte solution using the quaternary ammonium salt characteristically has low electric resistance and has great thermal stability, but is nevertheless likely to leak due to the mechanism described below.
Therefore, the quaternary ammonium salt used as the electrolyte solution in the electrolytic capacitor induces the deterioration of the electrical performance of the electrolytic capacitor, such as the reduction of the capacity of the electrolytic capacitor, due to the leakage of the electrolyte solution, so that the life of the electrolytic capacitor is shortened, disadvantageously.
Herein, the leakage of the electrolytic solution using quaternary ammonium salt is now described below. More specifically, leakage current occurs between the anode electrode foil 21 and the cathode electrode foil 22 under the application of direct voltage, because of the damage of the oxide film formed on the surface of the anode electrode foil 21, in such general-type electrolytic capacitor. Due to the occurrence of such leakage current, the reduction of dissolved oxygen or hydrogen ion occurs on the cathode side, with the resultant increase of the concentration of hydroxide ion in the interface between the electrode on the side of the cathode and the electrolyte solution. The phenomenon occurs both on the cathode electrode foil 22 and the lead terminal 10 for drawing the cathode; the increase of hydroxide ion, namely the increase of basicity, is observed in the proximity of the lead terminal, in particular. As the basicity increases in such manner, the sealing body 40 in contact to the lead terminal 10 is progressively damaged, involving the deterioration of the close contact between the lead terminal 10 and the sealing body 40, so that the hydroxide solution highly basic supposedly leaks outside.
As shown in FIG. 24, in other words, the leakage current of the electrolytic capacitor is expressed as the sum of the current I2 flowing in the cathode electrode foil 22 and the current I1 flowing in the lead terminal 10 for drawing the cathode, at the cathode side. Because the spontaneous potential E1 of the lead terminal 10 for drawing the cathode is generally at a nobler electric potential than the spontaneous potential E2 of the cathode electrode foil 22, the reduction of dissolved oxygen or hydrogen ion occurs due to the current initially flowing in the lead terminal 10 when the cathode side is cathode depolarized at a state loaded with direct current. Then, a current never consumed up for the reduction of dissolved oxygen or hydrogen ion on the lead terminal 10 flows in the cathode electrode foil 22, which induces reduction on the cathode electrode foil 22.
In this case, the active surface area of the cathode electrode foil 22 is so larger than the active surface area of the lead terminal 10 that the depolarization resistance of the cathode electrode foil 22 is smaller than the depolarization resistance of the lead terminal 10. Thus, at the voltage Excfx84 serving as the rated value Ixcfx84 of the leakage current of the electrolytic capacitor, the current I1 flows even in the lead terminal 10, although the current I2 flowing in the cathode electrode foil 22 is larger.
At a state loaded with direct current, therefore, the lead terminal 10 is continuously retained at a state in the flow of electric current, so that the reduction of dissolved oxygen or hydrogen ion consistently occurs on the surface of the lead terminal 10. Then, the resulting basic hydroxide ion incurs the decrease of the sealing precision.
When the electrolytic capacitor is left under no load, a local battery is constituted between the lead terminal for drawing the cathode and the cathode foil, because the spontaneous immersion potential E1 is higher than the spontaneous immersion potential E2 of the cathode foil, so that the reduction of dissolved oxygen or hydrogen ion occurs at the side of the lead terminal. At the state under no load, consequently, the resulting basic hydroxide ion incurs the decrease of the sealing precision.
As the electrolyte solution for driving electrolytic capacitor, with which the capacitor element 1 is impregnated, use is also made of an electrolyte solution dissolving a salt comprising an acid-conjugated base as the anion component and quaternary-prepared cyclic amidinium as the cation component (PCT/JP94/02028) in the principal solvents of xcex3-butyrolactone and ethylene glycol.
Because the hydroxide ion generated via the reduction of dissolved oxygen or hydrogen ion due to the occurrence of leak current as described above reacts with the quaternary-prepared cyclic amidinium and is thereby eliminated in case of the electrolyte solution dissolving the quaternary-prepared cyclic amidinium, it has been considered that the leakage of the electrolyte solution can be prevented.
As the outcome of the investigations made by the present inventors, however, it is revealed that the reaction of hydroxide ion with quaternary-prepared cyclic amidinium cannot progress completely when the interface between the electrode on the side of the cathode and the electrolyte solution is at a pH value of 12 or less, so that the hydroxide ion still remains. Under specific conditions, therefore, the leakage of the electrolyte solution dissolving quaternary-prepared cyclic amidinium salt cannot be suppressed completely as in the case of the electrolyte solution using quaternary ammonium salt, so that the resulting highly basic hydroxide solution leaks outside.
As has been described above, liquid leakage from electrolytic capacitors using quaternary ammonium salt and quaternary-prepared cyclic amidinium salt cannot be suppressed completely so that the resulting highly basic hydroxide solution leaks outside. When elastic rubber is used in the sealing body, accordingly, the sealing body is severely deteriorated, leading to poor air tightness.
(5) Objects of the Invention
The present invention has been proposed so as to overcome the problems of the conventional techniques. One object thereof is to provide a great electrolytic capacitor with a stably high life profile and possible contribution to the enhancement of productivity and the reduction of production cost, by improving the sealing structure. Another object is to provide a great method for efficiently producing such great electrolytic capacitor at low cost.
In addition, another object of the invention is to provide a great electrolytic capacitor using quaternary ammonium salt and quaternary- prepared amidinium salt, with a stably high life profile and possible contribution to the enhancement of productivity and the reduction of production cost, by improving the sealing structure to prevent the deterioration of the sealing body due to the electrolyte solution. A still additional purpose is to provide a great method for efficiently producing such great electrolytic capacitor at low cost.
So as to overcome the problems, in accordance with the present invention, an electrolytic capacitor with an improved sealing structure is provided, the capacitor comprising a capacitor element formed by winding both anode and cathode electrode foils connected with electrode drawing units via a separator, an outer case in a bottomed cylindrical shape for placing the capacitor element therein, and a sealing unit fitted on the opening of the outer case, wherein the electrode drawing unit comprises a plain portion for inner connection, a round rod portion for passing through the sealing unit, and an external connection portion and wherein the electrode drawing unit is drawn through a through-hole provided on the sealing unit outside the outer case; and an improved method for producing such electrolytic capacitor is also provided.
In accordance with the present invention, an electrolytic capacitor is provided, wherein a tube for retaining air tightness is placed between the through-hole of the sealing unit and the electrode drawing unit in a movable manner relative to the through-hole. When the round rod portion of the electrode drawing unit is inserted in the through-hole of the sealing unit during the process of producing the electrolytic capacitor, the tube placed between the round rod portion and the through-hole elongates along the direction of the insertion thereof due to the pressure from the electrode drawing unit because the tube is not bonded to the through-hole of the sealing unit with any adhesive or the like, so that the stress is dispersed to enhance the close contact between the tube and the round rod portion and the close contact between the tube and the through-hole. Because sufficient elasticity can be attained due to such elongation of the tube along the insertion direction, the elasticity can be utilized to readily insert even a tube with low gas permeability and a high elasticity modulus (high rigidity). Depending on the material of the tube, therefore, the gas permeability of the through-part of the electrode drawing unit can be reduced sufficiently and the air tightness of the part can be improved satisfactorily. Additionally, such tube can be produced readily and efficiently by series production.
In a first aspect of the inventive electrolytic capacitor, a tube made of a flexible material is fitted in a movable manner on the outer periphery of the round rod portion of the electrode drawing unit; and additionally, a portion including at least the through-hole of the sealing unit is made of a rigid material more rigid than the tube; and the round rod portion fitted with the tube is inserted in the through-hole of the sealing unit body. Because the sealing unit is made of a rigid material more rigid than the tube, in the electrolytic capacitor, the electrolytic capacitor is at low gas permeability. By using the tube made of a flexible material, furthermore, the tube can elongate well along the insertion direction due to the pressure loaded during the insertion of the electrode drawing unit into the through- hole, so that the air tightness of the through-part of the electrode drawing unit can be enhanced sufficiently.
In a second aspect of the inventive electrolytic capacitor, a tube made of a flexible material is fitted in a movable manner in the through-hole of the sealing unit made of a rigid material. The round rod portion of the electrode drawing unit is inserted in the tube. Even in the electrolytic capacitor, the tube can elongate well along the insertion direction due to the pressure during the insertion of the electrode drawing unit into the through-hole, as in the case of the tube fitted on the side of the electrode drawing unit, so that the air tightness of the through-part of the electrode drawing unit can be improved sufficiently.
A third aspect of the inventive electrolytic capacitor is characteristic in that a tube made of an alkali-resistant resin is fitted in a movable manner on the outer periphery of the round rod portion of at least the electrode drawing unit on the side of the cathode in case that an electrolyte solution containing quaternary ammonium salt or quaternary-prepared amidinium salt is used and that the round rod portion fitted with the tube is inserted in the through-hole provided on the sealing unit. Because the tube made of an alkali-resistant resin is fitted on the round rod portion of at least the electrode drawing unit on the side of the cathode, in the electrolytic capacitor, the deterioration of the through-part of the electrode drawing unit at the side of the cathode, where basicity is likely to increase, can be prevented so effectively that the air tightness between the round rod portion and the sealing unit can be enhanced. Preferably, the sealing unit is made of an alkali-resistant resin, thereby, even in case that highly basic hydroxide solutions are generated, the deterioration of the sealing unit per se can be prevented, with the resulting higher enhancement of the air tightness. Because the tube is fitted in a movable fashion on the round rod portion of the electrode drawing unit, as in the case of the aforementioned electrolytic capacitor, the tube can elongate well along the insertion direction, owing to the pressure loaded during the insertion of the electrode drawing unit in the through-hole. Thus, the air tightness of the through-part of the electrode drawing unit can be improved satisfactorily.
In the plural aspects of the electrolytic capacitor, preferably, the inner diameter of the tube is smaller than the outer diameter of the round rod portion of the electrode drawing unit. Similarly, the outer diameter of the tube is preferably larger than the diameter of the through-hole of the sealing unit, when the tube has been fitted on the round rod portion of the electrode drawing unit. Owing to the dimensional relation thereof, the close contact among the members can be enhanced, so the air tightness of the through-part of the electrode drawing unit can be enhanced more greatly.
Additionally, preferably, the diameter of at least the opening end on the side of the insertion inlet of the electrode drawing unit is larger than the diameter of the central part thereof. Thereby, the round rod portion of the electrode drawing unit can readily be inserted; and at least at the central part of the through-hole, the outer periphery face of the tube is secured to closely contact with the inner periphery face of the sealing unit.
Additionally, the sealing unit preferably comprises a sealing plate and an outer member made of a flexible material fitted on the outer periphery of the sealing plate. Even when the sealing plate is more or less inconsistent dimensionally, the elasticity of the outer periphery member made of a flexible material can absorb the dimensional error, so that the outer periphery face of the sealing unit is secured to closely contact with the inner periphery face of the opening of the outer case. Furthermore, preferably, the outer periphery member of the sealing unit is formed in a tube-like shape, while the inner diameter of the outer periphery member is smaller than the outer diameter of the sealing plate. The air tightness of the outer periphery part of the sealing structure can thereby be more enhanced, due to the elevation of the close contact between the sealing plate and the outer periphery member.